Boosting Neutral Water Oxidation through Surface Oxygen Modulation

Abstract

Developing efficient electrocatalysts for oxygen evolution reaction (OER) in pH-neutral electrolyte is crucial for microbial electrolysis cells and electrochemical CO₂ reduction. Unfortunately, the OER kinetics in neutral electrolyte is sluggish due to the low concentration of adsorbed reactants, with overpotentials of neutral OER at present much higher than that in acidic or alkaline electrolyte. Here, hydrated metal cations (Ca²⁺ ) are sought to be incorporated into the state-of-the-art Ru-Ir binary oxide to tailor the surface oxygen environments (lattice-oxygen and adsorbed oxygen species) for efficient neutral OER. Using a sol-gel method, ternary Ru-Ir-Ca oxides are synthesized in atomically homogenous manner, and the obtained catalyst on glassy carbon electrode achieves 10 mA cm^-2 at a low overpotential of 250 mV, with no degradation for 200 h of operation. In situ X-ray absorption spectroscopy, in situ ¹⁸ O isotope-labeling differential electrochemical mass spectrometry, and ¹⁸ O isotope-labeling secondary ion mass spectroscopy studies are carried out. The results reveal that incorporation of Ca²⁺ can enhance the covalency of metal-oxygen bonds and the electrophilic nature of surface metal-bonded oxygen sites; and simultaneously facilitate the adsorption of water molecules on catalyst surface, which accelerates the lattice-oxygen-involved reaction, thus improving the overall OER performance of RuIrCaO_x catalyst.